Half-bridges are used to switch loads to an upper supply voltage or to a lower supply voltage. In this case, a connection of the load is connected to a node which is common to the first and second switches. Half-bridges are used, for example, for motor control, for switched-mode power supplies and for lamp ballast circuits.
In order to avoid electromagnetic radiation emissions and to improve the switching response, so-called snubber arrangements are often inserted. The changing-over of the half-bridge from the first switch to the second switch and vice versa gives rise to the evolution of heat in the switches if the charge of capacitive loads is reversed. Such switching operations generally give rise to great switch-on losses in the switches involved. Therefore, the switches must be configured in such a manner that these switch-on losses in the switches can be converted into heat without destroying the switches in the process.
Switching losses can be reduced by means of so-called “zero voltage switching” (ZVS). Therefore, the switches of half-bridges which use zero voltage switching can be smaller and thus more cost-effective since these switches convert lower switching losses. If a half-bridge designed in this manner does not switch at the zero crossing, this is an undesirable operating mode of the half-bridge which must be detected so that measures which protect the switches of the half-bridge from thermal overloading can be initiated.
In particular in half-bridge circuits for lamp ballasts, the load of the half-bridge circuit is usually tuned in such a manner that it contains an inductive component. A capacitor (snubber) can be arranged in parallel with the half-bridge output. A dead time is inserted between the switched-on durations of the first and second switches. The dead time, capacitor and inductive component of the load current are usually tuned to one another in such a manner that the load current during the dead time suffices to reverse the charge of the capacitor. During the subsequent switch-on operation of a switch of the half-bridge circuit, the voltage of said switch is already zero, with the result that no switch-on losses occur. This operating mode is also referred to as “zero voltage switching” (ZVS).
If the inductive component of the load current does not suffice to completely reverse the charge of the capacitor, there is an incorrect response of the load. In this case, the voltage across the switch is not zero during the switch-on operation and the residual energy of the capacitor is converted into heat during the switch-on operation.
If the heat dissipation of the half-bridge switches is not designed for continuous non-zero-voltage switching operation, a circuit for detecting this operating state should be provided in order to switch off the half-bridge circuit, if necessary after a delay time. The delay time may be in the range from a few milliseconds to several seconds.
As a result of an incorrect response of the load, there are undesirable operating states in which the load current flows in the opposite direction during the dead time. Such an incorrect response of the load occurs, in the case of a lamp ballast circuit which normally operates in zero-voltage switching operation, for example when a lamp connected thereto is pulled from the holder or shattered during operation. In this case, the charge of the capacitor is not reversed and the load current flows during the dead time via the inverse diode of that switch of the half-bridge circuit which was switched off last. If the other switch of the half-bridge circuit is then switched on, this switch must not only convert the full amount of energy still stored in the capacitor into heat but must also eliminate the charge stored in the inverse diode of the switch which was turned off last. During the storage time, that switch of the half-bridge circuit which is just being switched on operates against a virtual short circuit to the full supply voltage. The heat loss produced in this case considerably exceeds the heat occurring during non-zero-voltage switching operation. This fault must be distinguished from the other faults since the half-bridge circuit must be switched off in a considerably more rapid manner. This fault is referred to below as hard commutation of the half-bridge circuit.